Conventional image recording apparatus used in general printing, photographic printing, and copying arts use light-emitting elements including laser diodes (LD) and light-emitting diodes (LED).
A color image is generally produced by using three chrominance signals of R, G and B, controlledly turning on light sources in correspondence with the chrominance signals to produce a light output, and scanning a color photosensitive material with the light output, followed by development and fixing. A monochromatic image can be similarly produced except that luminance signals and a monochromatic photosensitive material are used. To obtain images of high quality, it is desirable to optimize the exposure or exposure energy to the photosensitive material for each picture element.
One system contemplated for controlling an exposure per picture element, which relies on analog processing, is by amplifying each chrominance signal (a luminance signal for a monochromatic image) to modulate a light output of a light source. FIG. 6 is a diagram illustrating the light output (P) of a light-emitting element as a function of driving electric current (I). FIG. 7 is a diagram illustrating the image density (D) of a photosensitive material as a function of exposure energy (E). As seen from FIG. 6, the light-emitting element has a non-linear input-to-output (current-to-light output) relationship. As seen from FIG. 7, the photosensitive material also has a non-linear input-to-output (exposure-to-image density) relationship. Therefore, analog processing is unsuitable to produce images of high quality because of complexity and difficulty to effect correction between each chrominance signal and the photosensitive material image density.
Also contemplated is a digital processing system comprising sampling each chrominance signal for every picture element to convert it into a digital signal, and turning on a light source in response to the digital signal value, achieving pulsed exposure. This system includes a time interval modulation or pulse width modulation (PWM) mode in which continuous exposure is carried out for only a time corresponding to the digital signal value for every picture element, with the light output set fixed; an intensity modulation or pulse amplitude modulation (PAM) mode in which exposure is carried out with a light output corresponding to the digital signal value for every picture element, with the exposure time set fixed; and an exposure number modulation or pulse number modulation (PNM) mode in which exposure is carried out a number of times corresponding to the digital signal value for every picture element, with the light output and the single exposure time set fixed.
However, since light-emitting elements such as LD and LED largely vary their light output level with a change of temperature as shown in FIG. 6, control is usually carried out to compensate for such a variation. One conventional light output correction technique uses an automatic power control (APC) circuit.
The control method relying on the APC circuit maintains a certain selected light output level irrespective of temperature changes. Then, proper correction is not carried out at all levels of light output in the case of a light-emitting element having an essentially nonlinear input vs. output characteristic. In any exposure systems of the intensity, time interval and exposure number modulation modes, there would be left levels which are not corrected to a proper light output. This results in over- or under-exposure at corresponding areas, causing unevenness of exposure.
Where a laser diode is used as the light source, it changes its light emitting mode between laser oscillation and LED light emission in proximity to the threshold current, that is, changes its optical nature. Because of this nature and the previously mentioned variation of the output-to-input characteristic with temperature, modulation of the intensity at this light output level will result in an unstable light output, particularly in the case of PAM.
The photosensitive material has the nature that the difference between exposures (exposure energies) relative to a density difference is smaller in a moderate density zone as shown in FIG. 7. Thus, the time interval modulation mode as typified by PWM and the exposure number modulation mode as typified by PNM require a clock of a higher frequency as the number of gradations to be adjusted increases.
FIG. 8 is a diagram showing the effective sensitivity of a photosensitive material as a function of exposure time. It is seen from FIG. 8 that the effective sensitivity is lowered by reciprocity law failure when the exposure time is short. Differently stated, even when the exposure or exposure energy is kept constant, low intensity exposure or extremely high intensity short time exposure does not meet the reciprocity law that the optical density of an image is proportional to the exposure, that is, the product of light output and exposure time. In either case, reciprocity law failure occurs, failing to achieve a correct image density.
The intensity modulation or PAM and time interval modulation or PWM modes are affected by the reciprocity law failure at low or high light output levels. It is particularly true at low light output levels because the exposure time is short.